Objective lens element

ABSTRACT

An objective lens element which has excellent compatibility with optical discs having different base material thicknesses is provided. An objective lens element  141  has optically functional surfaces on an incident side and an exit side. Either one of the optically functional surfaces is divided into an inner part  131 B including a rotational symmetry axis and an outer part  131 F which is a ring-shaped region surrounding the inner part  131 B. On the inner part  131 B, a plurality of discontinuous steps are provided. The plurality of steps change in height in the same direction from the optical axis toward the outer part, and each of the steps causes a constant optical path difference longer than the wavelength λ 1  to the first incident light beam  61  and causes a constant optical path difference shorter than the wavelength λ 2  to the second incident light beam  62.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a U.S. continuation application filed under 35 USC111A claiming benefit under 35 USC 120 and 365C of PCT applicationJP2010/005694, filed Sep. 17, 2010, which claims priority to JapanesePatent Application No. 2009-216223, filed on Sep. 17, 2009. Theforegoing applications are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens element for use in anoptical head device that performs at least one of recording,reproducing, and erasing of information on an optical informationstorage medium.

2. Description of the Background Art

As media that record a large amount of information with a high density,there are optical information storage media such as optical discs.Optical discs record information as pit-shaped patterns thereon, and arewidely used for the purposes of recording digital audio files, videofiles, document files, and data files. Examples of functions requiredfor performing recording, reproducing, and erasing of information on anoptical disc with high reliability by using a light beam converged on amicro spot are a converging function to form a diffraction-limited microspot, focus control (focus servo) of an optical system, trackingcontrol, and pit signal (information signal) detection.

In recent years, due to advancement of optical system design technologyand shortening of the wavelengths of semiconductor lasers which arelight sources, development has progressed concerning optical discs thathave a higher-density storage capacity further than ever. One approachto density increase is to increase the optical disc-side numericalaperture (NA) of a light-converging optical system which converges alight beam to form a micro spot on the optical disc. However, when theNA of the light-converging optical system is increased, an amount of agenerated aberration increases with respect to a certain amount of tiltof the optical axis. In order to prevent this problem, it is necessaryto decrease the thickness of a layer (hereinafter, referred to as “basematerial thickness”) provided on a recording surface of the opticaldisc. In the present specification, the “base material thickness” meansa thickness from a light beam incident surface to an informationrecording surface of an optical disc.

For compact discs (CD) which are first generation optical discs,infrared light (a wavelength λ₃: 780 to 820 nm) and an objective lenshaving an NA of 0.45 are used. The base material thickness of CD is 1.2mm. For DVD which is second generation, red light (a wavelength λ₂: 630to 680 nm) and an objective lens having an NA of 0.6 are used. The basematerial thickness of DVD is 0.6 mm. For third generation optical discs,blue light (a wavelength λ₁: 390 to 415 nm) and an objective lens havingan NA of 0.85 are used. The base material thickness of third generationoptical discs is 0.1 mm. As described above, as the recording densityincreases, the base material thickness of the optical disc decreases.

In view of economical efficiency and space occupied by an apparatus, anoptical information recording/reproducing apparatus is desired which canperform recording and reproducing on optical discs having different basematerial thicknesses and recording densities. For this, alight-converging optical system which can converge a light beam to adiffraction limit on a recording surface of each of optical discs havingdifferent base material thicknesses, and an optical head deviceincluding this light-converging optical system, are necessary. Inaddition, when recording and reproducing are performed on an opticaldisc having a large base material thickness, it is necessary to convergea light beam on a recording surface located deeper than a beam incidentsurface of the optical disc, and thus the focal length has to beincreased.

Prior art documents disclose configurations intended for compatiblereproducing and compatible recording on an optical disc having a basematerial thickness of 0.6 mm and compatible with the wavelength λ₂ (redlight) and on an optical disc having a base material thickness of 0.1 mmand compatible with the wavelength λ₁ (blue light).

A first prior art example is a configuration in which awavelength-selective phase plate is combined with an objective lens.This is disclosed in Japanese Laid-Open Patent Publication No. 10-334504and the Proceedings of ISOM2001 (Session We-C-05), P30. Theconfiguration disclosed in the Proceedings of ISOM2001 (SessionWe-C-05), P30 will be described with reference to FIGS. 21 and 22. FIG.21 illustrates a schematic configuration of an optical head device.Parallel light emitted from a blue optical system 51 including a bluelight source of a wavelength λ₁ (405 nm) passes through a beam splitter161 and a wavelength selection phase plate 205 and is converged by anobjective lens 50 on an information recording surface of an optical disc10 (third generation optical disc) having a base material thickness of0.1 mm. The light reflected by the optical disc 10 travels along thereverse path and is detected with a detector of the blue optical system51. Diverging light emitted from a red optical system 52 including a redlight source of a wavelength λ₂ (650 nm) is reflected by the beamsplitter 161, passes through the wavelength selection phase plate 205,and is converged by the objective lens 50 on an information recordingsurface of an optical disc 10 (second generation optical disc: DVD)having a base material thickness of 0.6 mm. The light reflected by theoptical disc 10 travels along the reverse path and is detected with adetector of the red optical system 52.

The objective lens 50 is designed such that when the parallel light ofthe wavelength λ₁ is incident thereon, a convergence spot is formed at aposition where the light has passed through a protective layer having abase material thickness of 0.1 mm. When recording and reproducing areperformed on DVD, a spherical aberration occurs due to a difference inbase material thickness. In order to compensate the sphericalaberration, the light beam emitted from the red optical system 52 isadjusted to be diverging light, and the wavelength selection phase plate205 is used. When light incident on the objective lens is adjusted to bediverging light, a new spherical aberration occurs. Thus, the sphericalaberration occurring due to the difference in base material thicknesscan be cancelled by this new spherical aberration. Further, thewavefront is corrected also by the wavelength selection phase plate 205.

FIGS. 22A and 22B are a plan view and a cross-sectional view of thewavelength selection phase plate 205. The phase plate 205 has phasesteps 205 a of heights h and 3h. Here, the refractive index with respectto the wavelength λ₁ is n₁, and h=λ₁/(n₁−1). When the light of thewavelength λ₁ is used, an optical path difference caused by the phasestep of the height h is the used wavelength λ₁ and corresponds to aphase difference of 2π. Thus, the optical path difference is the same asa phase difference of 0. Therefore, the phase steps 205 a do notinfluence the phase distribution of the light of the wavelength λ₁ andhence do not influence recording and reproducing on the optical disc 10.Meanwhile, when the light of the wavelength λ₂ is used, designing isperformed such that an optical path difference (h×(n₂−1) which isprovided by the step to the light of the wavelength λ₂ has a value otherthan an integral multiple of the wavelength (e.g., 0.6 times). Byutilizing the phase difference caused by the optical path difference,the aberration compensation described above is performed.

As a second prior art example, a configuration in which a refractiontype objective lens and a diffraction element are combined is disclosed.In Japanese Laid-Open Patent Publication No. 2004-071134, in an opticalhead device which performs recording or reproducing on a high-densityoptical disc by using an objective lens having a high NA, asawtooth-like diffraction element is used in order to be able to alsoperform recording or reproducing on conventional optical discs such asDVD. The sawtooth height is set such that when blue light is used, thelength of the optical path becomes 2λ, and 2nd order diffracted light isused. The sawtooth-like diffraction element emits 1st order diffractedlight when red light is incident thereon. The blaze direction is as in aconvex lens type, and chromatic aberration compensation of therefractive lens is performed. The diffraction order when red light isused is lower than the diffraction order when blue light is used. Thus,the sawtooth-like diffraction element serves as a concave lens for redlight, thereby providing an effect that the working distance can beincreased.

Further, Japanese Laid-Open Patent Publication No. 2004-071134 alsodiscloses a diffraction element shown in FIG. 23A. The diffractionelement shown in FIG. 23A has a stair-like cross-sectional shape and iscomposed of consecutive unit steps whose number is an integer number.Each unit step provides an optical path difference of about 1.25wavelengths to a light beam of the wavelength λ₁. Specifically, thewavelength λ₁ is 390 to 415 nm, and a stair shape is provided in whichone cycle consists of steps which are 0 times, 1 times, 2 times, and 3times that of the unit step in height from the outer side of thediffraction element toward the optical axis. With respect to blue light,as shown in FIG. 23B, the phase changes in the same direction as that ofthe stair shape and a convex lens effect is exerted. With respect to redlight, as shown in FIG. 23C, the phase changes in the direction oppositeto that of the stair shape and a concave lens effect is exerted. Thus,when blue light is used, a chromatic aberration compensation effect ofthe refractive lens is obtained. In addition, when red light is used, aneffect that the working distance (the interval between the objectivelens surface and the surface of an optical disc) can be increased isobtained due to the concave lens effect.

As a third conventional art example, a configuration in which a relaylens is inserted between an infrared light source and an objective lens,thereby also realizing compatibility with a first generation opticaldisc having a base material thickness of 1.2 mm, is disclosed inJapanese Laid-Open Patent Publication No. 2004-281034.

Japanese Laid-Open Patent Publication Nos. 10-334504 and 2004-071134merely disclose the method for compatibility with the above secondgeneration optical discs and the above third generation optical discs.In addition, Japanese Laid-Open Patent Publication No. 2004-281034discloses the method for compatibility with the above first generationoptical discs, but requires a relay lens.

Further, it is desired that an element that realizes compatibility isintegrally formed on the objective lens surface, in view of costreduction by decrease in number of parts. However, in the conventionalart described above, only the exemplary configuration, in which thephase plate or the diffraction element is provided independently of therefraction type objective lens, is disclosed, and there is nodescription about integrally forming an element, which realizescompatibility, on the objective lens surface.

Moreover, in order to produce objective lenses at low cost and in largequantities, the material of the objective lenses is preferably resinrather than glass. In general, the material cost of resin is low, and itis also possible to mold resin at a lower temperature than to moldglass. Thus, the mold can be used long and the molding time can beshortened. Therefore, by molding resin to produce objective lenses, themanufacturing cost can be reduced. However, the refractive index of ahigh-NA objective lens made of resin changes due to temperature change.The refractive index change causes the refractive power of the lenssurface to shift from a designed value, whereby a spherical aberrationoccurs. A lower-order aberration greatly deteriorates the quality of aninformation reproduction signal, and thus a 3rd order sphericalaberration is problematic.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an objective lenselement having excellent compatibility with optical discs havingdifferent base material thicknesses. In addition, it is another objectof the present invention to provide an objective lens element whosechange in spherical aberration amount is small even when theenvironmental temperature changes.

The present invention is directed to an objective lens element which hasoptically functional surfaces on an incident side and an exit side,which converges a first incident light beam of a wavelength λ₁ through abase plate having a thickness t₁ to form a spot, and which converges asecond incident light beam of a wavelength λ₂ longer than the wavelengthλ₁ through a base plate having a thickness t₂ larger than the thicknesst₁ to form a spot.

In the objective lens element, at least either one of the opticalfunction surfaces is a refractive surface which deflects the first andsecond incident light beams by refractive power all over the surface, isdivided into an inner part which includes a rotational symmetry axis andthrough which the first and second incident light beams thatsubstantially contribute to spot formation pass, and an outer part whichis a ring-shaped region surrounding the inner part and through whichonly the first incident light beam that substantially contributes tospot formation passes, and has a plurality of discontinuous steps on theinner part. The plurality of steps change in height in the samedirection from the optical axis toward the outer part, and each of thesteps causes a constant optical path difference longer than thewavelength λ₁ to the first incident light beam and causes a constantoptical path difference shorter than the wavelength λ₂ to the secondincident light beam.

Alternatively, in the objective lens element, at least either one of theoptical function surfaces is a refractive surface which deflects thefirst and second incident light beams by refractive power all over thesurface, is divided into an inner part which includes a rotationalsymmetry axis and through which the first and second incident lightbeams that substantially contribute to spot formation pass, and an outerpart which is a ring-shaped region surrounding the inner part andthrough which only the first incident light beam that substantiallycontributes to spot formation passes, and has a plurality ofdiscontinuous steps. Each of the plurality of steps causes an opticalpath difference which is an integral multiple of the wavelength λ₁, tothe first incident light beam at a predetermined environmentaltemperature. When an environmental temperature changes, tendency ofspherical aberration change caused by change in shape of the refractivesurface and tendency of spherical aberration change caused by change inoptical path difference provided by the steps which is caused by changein refractive index are opposite to each other.

According to the present invention, by the steps provided on theoptically functional surface, converging light of the wavelength λ₁ isfurther converged and converging light of the wavelength λ₂ is diffused.Thus, the focal point of the objective lens element can be changed so asto correspond to each of the base material thickness t₁ and t₂. Inaddition, an objective lens element whose change in spherical aberrationamount is small even when the environmental temperature changes whilelight of the wavelength λ₁ is used can be realized.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 1;

FIG. 1B is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 1;

FIG. 1C is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 1;

FIG. 2 is a diagram illustrating the objective lens element according toEmbodiment 1;

FIG. 3 is a diagram illustrating a mold used for manufacturing theobjective lens element shown in FIG. 2 and a method for manufacturingthe mold;

FIG. 4 is a diagram illustrating an example of a step structure formedon an outer part;

FIG. 5A is a diagram illustrating an example of a diffraction gratingformed on the outer part;

FIG. 5B is a diagram illustrating an example of a diffraction gratingformed on the outer part;

FIG. 5C is a diagram illustrating an example of a diffraction gratingformed on the outer part;

FIG. 6 is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 2;

FIG. 7 is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 3;

FIG. 8 is a diagram illustrating the objective lens element according toEmbodiment 3;

FIG. 9A is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 4;

FIG. 9B is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 4;

FIG. 9C is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 4;

FIG. 9D is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 4;

FIG. 10 is a diagram illustrating the objective lens element accordingto Embodiment 4;

FIG. 11 is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 5;

FIG. 12 is a diagram illustrating the objective lens element accordingto Embodiment 5;

FIG. 13 is a diagram when the objective lens shown in FIG. 12 is viewedfrom below;

FIG. 14 is a diagram illustrating an optical head device according toEmbodiment 6;

FIG. 15 is a diagram illustrating an optical information apparatusaccording to Embodiment 7;

FIG. 16 is a diagram illustrating a computer according to Embodiment 8;

FIG. 17 is a diagram illustrating an optical disc player according toEmbodiment 9;

FIG. 18 is a diagram illustrating an optical disc recorder according toEmbodiment 10;

FIG. 19 is a diagram illustrating an information server according toEmbodiment 11;

FIG. 20 is a diagram illustrating a vehicle according to Embodiment 12;

FIG. 21 is a diagram illustrating a schematic configuration of aconventional optical head device;

FIG. 22A is a plan view of an optical element shown in FIG. 21;

FIG. 22B is a cross-sectional view of the optical element shown in FIG.21;

FIG. 23A is a diagram illustrating a step structure provided on asurface of an objective lens element;

FIG. 23B is a diagram illustrating a step structure provided on asurface of an objective lens element; and

FIG. 23C is a diagram illustrating a step structure provided on asurface of an objective lens element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Compatibility withBD and DVD

FIG. 1 is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 1. FIG. 1A is a diagramillustrating the physical shape of the steps. It should be noted that inreality, the steps are formed on a base surface which is a refractivesurface (aspherical shape) in the surface of the objective lens element.FIG. 1B shows an amount of phase change provided by each step shown inFIG. 1A to light of a wavelength λ₁, and FIG. 1C shows an amount ofphase change provided by each step shown in FIG. 1A to light of awavelength λ₂ (λ₂>λ₁). As an example, the light of the wavelength λ₁ isblue light (390 to 415 nm) for BD, and the light of the wavelength λ₂ isred light (630 to 680 nm) for DVD.

In FIG. 1A, the vertical direction indicates the thickness of the lensmaterial in the optical axis direction, or the height thereof. Forexample, when a polyolefin resin is used as the material of theobjective lens element, an element material having a refractive indexn_(b) of about 1.522 with respect to first incident light can be used. Astep d₁ (hereinafter, also referred to as “unit step”) is dimensioned soas to cause an optical path difference of about 1.25 wavelengths, or aphase difference of about (2π+π/2), to the light of the wavelength λ₁.The unit step d₁ can be represented by the following equation.

d ₁=λ₁/(n _(b)−1)×1.25

It should be noted that the “optical path difference” means thedifference between the length of an optical path in the case where thesteps are present (the medium of the step portion is the lens material)and the length of an optical path in the case where no steps are present(the medium of the step portion is air).

Where the unit step is d₁, the height (level) from the base surface isan integral multiple of d₁. Thus, an amount of phase change provided bythe step shape to the light of the wavelength λ₁ is an integral multipleof (2π+π/2). This substantially means that the amount of phase changechanges by π/2 each time the height from the base surface increases byone step (d₁) as shown in FIG. 1B.

Among lens materials composed of a polyolefin resin, a lens materialhaving a refractive index n_(r) of about 1.505 with respect to the lightof the wavelength λ₂ is available. The optical path difference providedby the step d₁ to the light of the wavelength λ₂ can be represented byd₁×(n_(r)−1). Where λ₁ is 405 nm, λ₂ is 650 nm, n_(b) is 1.522, andn_(r) is 1.505, the optical path difference corresponds to about 0.75wavelength, and this means that the amount of phase change changes by−π/2 each time the height from the base surface increases by one step(d₁).

When the height of each step structure from the base surface is anintegral multiple of d₁ and a stair-like cross-sectional shape isprovided as shown in FIG. 1A, the amount of phase change provided to thelight of the wavelength λ₁ changes by π/2 per step as shown in FIG. 1B.In other words, the optical path difference changes toward the opticalaxis of the objective lens element in steps of +¼ wavelength.

Meanwhile, as shown in FIG. 1C, the amount of phase change provided tothe light of the wavelength λ₂ changes by −π/2 per step. In other words,the optical path difference changes toward the optical axis of theobjective lens element in steps of −¼ wavelength. The amount of phasechange is positive when the light of the wavelength λ₁ is used, whilethe amount of phase change is negative when the light of the wavelengthλ₂ is used. This means that the light of the wavelength λ₁ and the lightof the wavelength λ₂ are subjected to the opposite actions from thesteps.

Here, when the steps are formed such that the height from the basesurface increases toward the inner side of the objective lens element asshown in FIG. 1A, the optical path of the light of the wavelength λ₁ islengthened toward the inner side. On the other hand, the optical path ofthe light of the wavelength λ₂ is shortened toward the inner side. Inother words, when the intervals at which the phase steps are formed areappropriately set and the objective lens element is configured to exerta convex lens effect on the first incident light, a concave lens effectis exerted on second incident light. Then, the focal point of the firstincident light gets close to the objective lens element, while the focalpoint of the second incident light moves away from the objective lenselement. Thus, an effect is obtained that the second incident light canbe converged on an information recording surface through a thicker basematerial.

In the present embodiment, the steps shown in FIG. 1A are formed in astair shape in which the height monotonically increases or decreases.Thus, the step structure in the present embodiment does not serve as adiffraction element having a periodic structure in which the heightincreases and decreases every certain cycle (see FIG. 23A), but servesas an element which changes a phase in a stair-like manner.

FIG. 2 is a diagram illustrating the objective lens element according toEmbodiment 1.

On an optically functional surface of the objective lens element 141 onan incident side, an inner part 131B including a rotational symmetryaxis and a ring-shaped outer part 131F surrounding the inner part 131Bare provided. When the light of the wavelength λ₁ is used, the objectivelens element 141 converges first incident light 61 incident on both theinner part 131B and the outer part 131F, and forms a spot on aninformation recording surface 91 of an optical disc 9 through a basematerial having a thickness t₁. In addition, when the light of thewavelength λ₂ is used, the objective lens element 141 converges secondincident light 62 incident on the inner part 131B, and forms a spot onan information recording surface 101 of an optical disc 10 through abase material having a thickness t₂. The outer part 131F is a regionwhich substantially does not contribute to spot formation when the lightof the wavelength λ₂ is used. Where the numerical aperture defined whenthe first incident light 61 is converged on the information recordingsurface 91 of the optical disc 9 is NA₁ and the numerical aperturedefined when the second incident light 62 is converged on theinformation recording surface 101 of the optical disc 10 is NA₂, NA₁ isequal to or higher than 0.85 and NA₂ is equal to or higher than 0.6.

In other words, the inner part 131B is a region shared by the light ofthe wavelength λ₁ and the light of the wavelength λ₂, and the outer part131F is a region dedicated for the light of the wavelength λ₁. On theinner part 131B, the stair-like step structure shown in FIG. 1A isformed. The outer part 131F can be configured as in any of (1) to (4)below.

(1) On the outer part 131F as well, the same step structure as thatformed on the inner part 131B is formed. In this case, the outer part131F suffices to be designed such that when the light of the wavelengthλ₁ is used, the light incident on the outer part 131F is converged onthe information recording surface 101 through the base material havingthe thickness t₁, and when the light of the wavelength λ₂ is used, thelight incident on the outer part 131F generates a great aberration at aposition where the light has passed through the base material having thethickness t₂ and is substantially not converged on the informationrecording surface 91.

(2) Alternatively, no steps may be formed on the outer part 131F, andthe outer part 131F may be designed such that when the light of thewavelength λ₁ is used, the light incident on the outer part 131F isconverged on the information recording surface 101 through the basematerial having the thickness t₁, and when the light of the wavelengthλ₂ is used, the light incident on the outer part 131F generates a greataberration at a position where the light has passed through the basematerial having the thickness t₂ and is substantially not converged onthe information recording surface 91.

(3) Still alternatively, steps each of which causes an optical pathdifference of 1 wavelength to the first incident light of the wavelengthλ₁ may be formed on the outer part 131F. In this case, a phasedifference substantially does not occur in the light of the wavelengthλ₁ having passed through the outer part 131F, and thus the light of thewavelength λ₁ incident on the outer part 131F is converged on theinformation recording surface 101 through the base material having thethickness t₁. Meanwhile, each of these steps provides a phase differencewhich is about 0.6 times of the wavelength λ₂, to the light of thewavelength λ₂. Thus, a great aberration occurs and the light of thewavelength λ₂ incident on the outer part 131F is substantially notconverged on the information recording surface 91.

(4) Still alternatively, a sawtooth-like diffraction grating may beformed on the outer part 131F, and designing may be performed such thatthe light of the wavelength λ₁ incident on the outer part 131F isconverged on the information recording surface 101 through the basematerial having the thickness t₁ and the light of the wavelength λ₂incident on the outer part 131F is substantially not converged on theinformation recording surface 91. In this case, the sawtooth height ispreferably set to such a height that an optical path difference of 1wavelength is caused to the light of the wavelength λ₁ as shown in FIG.5A. When the sawtooth height is set to a height equal to 1 wavelength ofthe light of λ₁ as shown in FIG. 5B, the diffraction efficiency of the1st order diffracted light increases. Thus, the light of the wavelengthλ₁ incident on the outer part 131F can be converged through the basematerial having the thickness t₁. Meanwhile, with respect to the lightof the wavelength λ₂, the sawtooth height is only about 0.6 times ofwavelength as shown in FIG. 5C. Thus, the light of the wavelength λ₂incident on the outer part 131F is diffracted into 1st order diffractedlight and zero order diffracted light and is not converged on a point,and the outer part 131F exerts an aperture limiting function.

As in the present embodiment, a monotonically-increasing type ormonotonically decreasing type stair-like step structure is suitable tobe integrally formed directly on the surface of the objective lenselement 141. The reason will be described with reference to FIG. 3.

FIG. 3 is a diagram illustrating a mold used for manufacturing theobjective lens element 141 shown in FIG. 2 and a method formanufacturing the mold.

A concavity of the mold 1401 shown in FIG. 3 is a surface for shapingthe incident-side surface of the objective lens element 141 shown inFIG. 2. In order to integrally form the above steps on the surface ofthe objective lens element 141, ring-shaped recessed and raised portionscorresponding to the shape of the steps are formed on the concavity ofthe mold 1401. When the height of the steps increases toward the innerside as shown in FIG. 1A, the ring-shaped recessed portions formed onthe concavity of the mold 1401 suffices to be deeper on the inner side.For this mold shape, machining is possible by cutting with a cutter1402. Cutting is performed with rotation of the mold 1401 about acentral axis 1401C of the mold 1401, to form concentric ring zones onthe concavity of the mold 1401.

The cutter 1402 has a surface 1402A and an inclined surface 1402B whichmakes a predetermined angle with respect to the surface 1402A, and itsthickness decreases toward its pointed end. During cutting, the surface1402A is located so as to extend upwardly and substantiallyperpendicularly from a portion where the cutter 1402 contacts the mold1401, and the steep step shape on the outer side is cut. The inclinedsurface 1402B is located so as to be inclined toward the inner side. Inorder to prevent the cutter 1402 from being chipped or broken, thecutter 1402 has to have a certain level of strength. For this, theinclined surface 1402B is desirably provided such that the thicknessincreases with increasing distance from the pointed end. In the exampleof FIG. 3, the inclined surface 1402B is inclined toward the inner side,and no problem occurs in terms of cutting, since the ring-shapedrecessed portions formed on the concavity of the mold 1401 aremonotonically deepened toward the inner side. Meanwhile, when the stepstructure shown in FIG. 23A is formed, it is necessary to form, on theconcavity of the mold, ring-shaped recessed portions whose depthperiodically increases and decreases. Thus, cutting with a tool as shownin FIG. 3 is difficult. On the other hand, the thickness monotonicallychanges in the stair-like structure in the present embodiment, and thusmachining can be realized as designed.

Embodiment 2 Temperature Compensation of Resin Objective Lens

As a material forming an objective lens element, glass and resin(plastic) are often used. Glass changes less in properties with respectto temperature change, and thus the performance thereof can stably beobtained. Meanwhile, resin can be changed in shape even at a lowtemperature as compared to glass, and thus the manufacturing cost can bereduced. In addition, resin has an advantage of being lightweight.However, resin has a drawback in that its change in refractive indexwith temperature change is greater by 10 times than that of glass.Particularly, when an objective lens having a high numerical aperture isformed from resin, the refractive index changes with temperature change,and thus the spherical aberration changes, which is a problem.

In a system for optical disc called Blue-Ray Disc (BD) employing bluelight, the numerical aperture (NA) is set so as to be equal to or higherthan 0.85, and information reproducing is realized with a high recordingdensity. When an objective lens for BD having a numerical aperture of0.85 or higher is formed by a resin lens, it is desirable to reducechange in spherical aberration at temperature change. Particularly, itis desirable to separate a spherical aberration into each order and toreduce a 3rd order spherical aberration. The lowest order sphericalaberration is caused by a power component, namely, a lens effect, and isreduced by a focus control mechanism of an optical pickup in a shorttime of about 1 ms. The optical pickup for BD includes means forcompensating a 3rd order spherical aberration but often does not includemeans for directly detecting a 3rd order spherical aberration. This isbecause in order to obtain an amount of change in 3rd order sphericalaberration as a spherical aberration detection signal detected within 1ms, a detection optical system of the optical pickup including aphotodetector is complicated, leading to size increase and costincrease. In general, a 3rd order spherical aberration is oftencompensated by driving the spherical aberration compensating means inaccordance with output of a temperature sensor provided in the opticalpickup. In other words, since a spherical aberration is not directlydetected and open loop control is performed in accordance with output ofthe temperature sensor, real-time compensation is not performed within 1ms but low-accurate compensation is performed for a time period longerthan 1 ms. Thus, it is desirable to reduce an amount of change in 3rdorder spherical aberration with temperature change. Specifically, theamount of change in 3rd order spherical aberration per temperaturechange of 30° C. is desirably equal to or less than 70 mλ. 70 mλ or λ/14corresponds to so-called Marechal Criterion. The amount of change in 3rdorder spherical aberration per temperature change of 30° C. is morepreferably equal to or less than 60 mλ. In addition, the amount ofchange in 5th order spherical aberration per temperature change of 30°C. is preferably equal to or less than 30 mλ.

FIG. 6 is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 2.

As an example, an objective lens having a focal length of 1.3 mm and anumerical aperture of 0.86 was produced from a polyolefin resin. At thattime, when the temperature changes by 30° C., a 3rd order sphericalaberration change of about 130 mλ occurred. Thus, as shown in FIG. 6,steps each of which causes an optical path difference which is anintegral multiple of the wavelength λ₁ at a specific environmentaltemperature, are provided on a surface of an objective lens element.FIG. 6 illustrates the case where the optical path difference caused byeach step is 5 times of the wavelength λ₁, but the present invention isnot particularly limited to 5 times. Being an integral multiple is acondition, and numerical values other than 5 times, such as 6 times and4 times, are also possible depending on the designing conditions.

Where the physical height of each step is d₅, the optical pathdifference caused by the step is represented by d₅×(n₅−1). Here, n₅ isthe refractive index of the lens material. When the refractive index ofthe lens material changes due to temperature change, the optical pathdifference caused by the step also changes. This change is utilized toreduce a spherical aberration caused by change in angle of refraction ata refractive surface, particularly, a 3rd order spherical aberration.Specifically, the steps are designed such that when the environmentaltemperature changes, tendency of change in spherical aberration causedby change in refractive surface shape is opposite to tendency of changein spherical aberration caused by change in optical path differenceprovided by the steps which is caused by change in refractive index. Asa result, when the environmental temperature changes, a sphericalaberration caused by change in refractive surface shape and a sphericalaberration caused by change in optical path difference provided by thesteps which is caused by change in refractive index cancel each other,whereby an aberration is reduced.

In order to reduce a spherical aberration caused by temperature changewhen the light of the wavelength λ₁ is used, the thickness of the stepsshown in FIG. 6 desirably monotonically increases or monotonicallydecreases in the same direction. When being configured as describedabove, a mold for molding the objective lens element can easily beproduced as described in Embodiment 1.

Embodiment 3 Temperature Compensation of Resin Objective Lens andCompatibility with BD and DVD

The present embodiment realizes compatibility with BD and DVD asdescribed in Embodiment 1 and suppression of a spherical aberration inthe resin objective lens at temperature change as described inEmbodiment 2, at the same time. Here, compatibility means that a singleobjective lens element can form a spot on a recording surface throughthe base material having the thickness t₁ when the light of thewavelength λ₁ is used, and can form a spot on a recording surfacethrough the base material having the thickness t₂ when the light of thewavelength λ₂ is used.

FIG. 7 is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 3. FIG. 7( a) is adiagram illustrating the physical shape of steps which realize thecompatibility described in Embodiment 1. FIG. 7( b) is a diagramillustrating the physical shape of steps for realizing the sphericalaberration suppression described in Embodiment 2. Each of the stepsshown in FIG. 7( b) causes an optical path difference which is anintegral multiple of the wavelength λ₁, at a specific temperature asdescribed in Embodiment 2. FIG. 7( c) is a diagram illustrating a shapeobtained by combining the step shape of FIG. 7( a) and the step shape ofFIG. 7( b). In Embodiment 3, the step shape of FIG. 7( c) is formed on abase surface which is a refractive surface (aspherical shape) in thesurface of the objective lens element.

It should be noted that the steps of FIG. 7( b) which are components ofthe steps of FIG. 7( c) have differences from those according toEmbodiment 2 (FIG. 6). The optical path difference caused by each stepof FIG. 6 to the light of the wavelength λ₁ is an integral multiple ofthe wavelength λ₁, while the optical path difference caused by each stepof FIG. 7( b) to the light of the wavelength λ₁ is 5N times (N is aninteger other than 0) of the wavelength λ₁. At that time, each step ofFIG. 7( b) causes an optical path difference which is 3N times of thewavelength λ₂, to the light of the wavelength λ₂, and a great aberrationdoes not occur. Compatibility with the wavelength λ₁ and the wavelengthλ₂ is realized by the step shape of FIG. 7( a), and the steps of FIG. 7(b) are intended to suppress a spherical aberration caused by temperaturechange when the light of the wavelength λ₁ is used. Thus, it isdesirable that each step of FIG. 7( b) does not cause a great aberrationto the light of the wavelength λ₂. Due to this reason, the optical pathdifference caused by each step of FIG. 7( b) to the light of thewavelength λ₁ is desirably 5N times (N is an integer other than 0) ofthe wavelength λ₁.

Each of the step structure of the FIG. 7( a) and the step structure ofFIG. 7( b) is desirably a stair-like structure in which the thicknessmonotonically increases or monotonically decreases in the samedirection. When being configured as described above, the step structureof FIG. 7( c) is also a stair-like structure in which the thicknessmonotonically increases or monotonically decreases. Thus, a mold formolding the objective lens element can easily be produced.

FIG. 8 is a diagram illustrating the objective lens element according toEmbodiment 3.

On at least an inner part 132B, the stair-like structure shown in FIG.7( c) is formed. The reference characters 132 p and 132 q assigned tothe inner part 132B of FIG. 8 correspond to the steps shown in FIG. 7(b). When the light of the wavelength λ₁ is used, the objective lenselement 142 converges the first incident light 61 incident on both theinner part 132B and an outer part 132F, and forms a spot on theinformation recording surface 91 of the optical disc 9 through the basematerial having the thickness t₁. In addition, when the light of thewavelength λ₂ is used, the objective lens element 142 converges thesecond incident light 62 incident on the inner part 132B, and forms aspot on the information recording surface 101 of the optical disc 10through the base material having the thickness t₂. Where the numericalaperture defined when the first incident light 61 is converged on theinformation recording surface 91 of the optical disc 9 is NA₁ and thenumerical aperture defined when the second incident light 62 isconverged on the information recording surface 101 of the optical disc10 is NA₂, NA₁ is equal to or higher than 0.85 and NA₂ is equal to orhigher than 0.6.

In other words, the inner part 132B is a region shared by the light ofthe wavelength λ₁ and the light of the wavelength λ₂, and the outer part132F is a region dedicated for the light of the wavelength λ₁. On theinner part 132B, the stair-like step structure shown in FIG. 1A isformed. The outer part 132F can be configured as in (5) to (8) below.

(5) A step structure which is the same as that provided on the innerpart 132B is formed on the outer part 132F. In this case, the outer part132F suffices to be designed such that the light of the wavelength λ₁forms a spot on the information recording surface 91 through the basematerial having the thickness t₁ but the light of the wavelength λ₂causes a great aberration and is not converged on a position through thebase material having the thickness t₂.

(6) Alternatively, the steps of FIG. 7( a) may not be formed on theouter part 132F. In this case, the outer part 132F suffices to bedesigned such that the light of the wavelength λ₁ forms a spot on theinformation recording surface 91 through the base material having thethickness t₁ but the light of the wavelength λ₂ causes a greataberration and is not converged through the base material having thethickness t₂.

(7) Still alternatively, the steps shown in FIG. 4 each of which causesan optical path difference of 1 wavelength to the light of thewavelength λ₁ may be formed on the outer part 132F. In this case, withthe optical path difference of 1 wavelength of the wavelength λ₁, aphase difference substantially does not occur. Thus, similarly to thecase where no steps are formed, the light of the wavelength λ₁ isconverged through the base material having the thickness t₁, but a phasedifference of about 0.6 wavelength occurs in the light of the wavelengthλ₂ and hence the light of the wavelength λ₂ is not converged through thebase material having the thickness t₂.

(8) Still alternatively, a sawtooth-like diffraction grating may beformed on the outer part 132F. In this case, the outer part 132Fsuffices to be designed such that the light of the wavelength λ₁ forms aspot on the information recording surface 91 through the base materialhaving the thickness t₁ but the light of the wavelength λ₂ is notconverged through the base material having the thickness t₂. Further,the sawtooth height is preferably set to such a height that an opticalpath difference of 1 wavelength is caused to the light of the wavelengthλ₁ as described with reference to FIG. 5A. When the sawtooth height isequal to 1 wavelength of the wavelength λ₁ as shown in FIG. 5B, thediffraction efficiency of the 1st order diffracted light increases.Meanwhile, with respect to the light of wavelength λ₂, the sawtoothheight is only about 0.6 times of wavelength as shown in FIG. 5C. Thus,light is diffracted into 1st order diffracted light and zero orderdiffracted light and is not converged on a point. Therefore, an aperturelimiting effect on the light of the wavelength λ/incident on the outerpart 132F can be obtained.

(9) In order to suppress a spherical aberration caused by temperaturechange when the light of wavelength λ₁ is used, the steps shown in FIG.6 may be provided on the outer part 132F. In addition, the steps shownin FIG. 6 may be formed on any of the above shapes provided on the outerpart 132F. In this case, in order to facilitate production of a mold forthe objective lens element 142, it is desirable to monotonicallyincrease or monotonically decrease the thickness of the steps.

Embodiment 4 Compatibility with BD, DVD, and CD

An objective lens element according to the present embodiment convergeslight of a wavelength λ₃ through a base material having a thickness t₃,in addition to the compatibility with the wavelengths λ₁ and λ₂ asdescribed in Embodiment 1.

Here, t ₁ <t ₂ <t ₃, and

NA₁>NA₂>NA₃.

NA₃ is the numerical aperture defined when the light of the wavelengthλ₃ is converged. The light of the wavelength λ₃ is, for example,infrared light of 780 to 820 nm.

Since the numerical aperture NA₃ is the lowest, the innermost portion ofthe objective lens element is used for conversing the light of thewavelength λ₃ on an information recording surface. Thus, the innermostportion of the objective lens element has to be formed so as to be ableto converge light of three kinds of wavelengths through correspondingbase materials having thicknesses. For that reason, in the presentembodiment, a stair shape shown in FIG. 9A is formed on the innermostportion of the objective lens element.

FIG. 9 is a diagram illustrating steps formed on a surface of theobjective lens element according to Embodiment 4. FIG. 9A is a diagramillustrating the physical shape of the steps formed on the surface ofthe objective lens element according to Embodiment 4. The shape of FIG.9A is formed on a base surface which is a refractive surface (asphericalshape) in the surface of the objective lens element. FIG. 9B shows anamount of phase change provided by each step shown in FIG. 9A to thelight of the wavelength λ₁. FIG. 9C shows an amount of phase changeprovided by each step shown in FIG. 9A to the light of the wavelengthλ₂. FIG. 9D shows an amount of phase change provided by each step shownin FIG. 9A to the light of the wavelength λ₃.

In FIG. 9A, the vertical direction indicates the thickness of the lensmaterial in the optical axis direction, or the height thereof. Each stepis designed to have such a height that an optical path difference ofabout 1.25 wavelengths is provided to the light of the wavelength λ₁.The optical path difference of about 1.25 wavelengths corresponds to aphase difference of about (2π+π/2). Due to the steps, an amount of phasechange changes by π/2 per step as shown in FIG. 9B.

Further, due to the reason described in Embodiment 1, with respect tothe light of the wavelength λ₂, an amount of phase change changes byabout −π/2 per step (see FIG. 9C).

Moreover, with respect to the light of the wavelength λ₃, each stepprovides an optical path difference of about −0.375 wavelength. Due tothe steps, an amount of phase change changes by about −0.75π per step asshown in FIG. 9D.

The amount of phase change provided to the light of the wavelength λ₁ ispositive, while the amounts of phase change provided to the light of thewavelengths λ₂ and λ₃ are negative. This indicates that the steps shownin FIG. 9A exert an effect which is opposite to that on the light of thewavelength λ₁, on the light of the wavelengths λ₂ and λ₃.

When the thickness of the steps increase toward the inner side of theobjective lens element as shown in FIG. 9A, the optical path differenceof the light of the wavelength λ₁ is lengthened toward the inner side.Meanwhile, the optical path differences of the light of the wavelengthsλ₂ and λ₃ are shortened toward the inner side. When the intervals atwhich the phase steps are formed are designed as appropriate and thephase steps are configured to exert a convex lens effect on the light ofthe wavelength λ₁, a concave lens effect is exerted on red light andinfrared light. Then, the focal point of the light of the wavelength λ₁gets close to the objective lens element, while the focal points of thelight of the wavelengths λ₂ and λ₃ move away from the objective lenselement. Thus, the light of the wavelengths λ₂ and λ₃ can be convergedon information recording surfaces through base materials thicker thanthe thickness t₁.

In the present embodiment, the steps shown in FIG. 9A are formed in astair shape in which the height monotonically increases or decreases.Thus, the step structure in the present embodiment does not serve as adiffraction element having a periodic structure in which the heightincreases and decreases every certain cycle (see FIG. 23A), but servesas an element which changes a phase in a stair-like manner. In thiscase, a mold for molding the objective lens element can easily beproduced.

FIG. 10 is a diagram illustrating the objective lens element accordingto Embodiment 4.

On at least an inner part 133C, the stair shape shown in FIG. 9A isformed. When the light of the wavelength λ₁ is used, the objective lenselement 143 converges the first incident light 61 incident on the innerpart 133C, an intermediate part 133B, and an outer part 133F, and formsa spot on the information recording surface 91 of the optical disc 9through the base material having the thickness t₁. When the light of thewavelength λ₂ is used, the objective lens element 143 converges thesecond incident light 62 incident on the inner part 133C and theintermediate part 133B, and forms a spot on the information recordingsurface 101 of the optical disc 10 through the base material having thethickness t₂. When the light of the wavelength λ₃ is used, the objectivelens element 143 converges third incident light 63 incident on the innerpart 133C, and forms a spot on an information recording surface 111 ofan optical disc 11 through the base material having the thickness t₃.Here, NA₁ is equal to or higher than 0.85, NA₂ is equal to or higherthan 0.6, and NA₃ is equal to or higher than 0.45.

In other words, the inner part 133C is a region shared by the light ofthe wavelengths λ₁, λ₂, and λ₃, the intermediate part 133B is a regionshared by the light of the wavelengths λ₁ and λ₂, and the outer part133F is a region dedicated for the light of the wavelength λ₁. A regionwhich is a combination of the inner part 133C and the intermediate part133B corresponds to the inner part 131B shown in FIG. 1. The outer part133F corresponds to the outer part 131F shown in FIG. 1. Any of theabove-described exemplary configurations of the outer part can beapplied to the outer part 133F.

Embodiment 5 Temperature Compensation of Resin Objective Lens Elementand Compatibility with BD, DVD, and CD

The present embodiment realizes compatibility with the three wavelengthsas described in Embodiment 4 and suppression of a spherical aberrationcaused by temperature change as described in Embodiment 2, at the sametime.

FIG. 11 is a diagram illustrating steps formed on a surface of anobjective lens element according to Embodiment 5. Specifically, FIG. 11(a) is a diagram illustrating the physical shape of steps which realizecompatibility with the wavelengths λ₁, λ₂, and λ₃. FIG. 11( b) is adiagram illustrating the physical shape of steps for suppressing aspherical aberration caused by temperature change of the resin objectivelens. Each of the steps shown in FIG. 11( b) causes an optical pathdifference which is an integral multiple of the wavelength λ₁, at aspecific environmental temperature. FIG. 11( c) is a diagramillustrating steps obtained by combining the steps of FIG. 11( a) andthe steps of FIG. 11( b). The cross-sectional shape of FIG. 11( c) isformed on a base surface which is a refractive surface (asphericalshape) in the surface of the objective lens element.

In FIG. 11( b), there are differences from the steps shown in FIG. 6(Embodiment 2) and FIG. 7( b) (Embodiment 3). The optical pathdifference caused by each step is an integral multiple of the wavelengthλ₁ in the example of FIG. 6, and is 5N times (N is an integer other than0) of the wavelength λ₁ in the example of FIG. 7. In FIG. 11( b), theoptical path difference caused by each step is set so as to be 10M times(M is an integer other than 0) of the wavelength λ₁ as a condition. Thecondition is equal to limiting N to an even number, namely, N=2M, inEmbodiment 3. At this time, each step of FIG. 11( b) causes an opticalpath difference of 6M times of wavelength to the light of the wavelengthλ₂, and thus does not cause a great aberration. Further, each step ofFIG. 11( b) causes an optical path difference of 5M times of wavelengthto the light of the wavelength λ₃, and thus does not cause a greataberration. The compatibility with the three wavelengths is realizedwith the step shape shown in FIG. 11( a), and the steps shown in FIG.11( b) are intended to suppress a spherical aberration caused byenvironmental temperature change when the light of the wavelength λ₁ isused. Thus, it is desirable that the steps shown in FIG. 11( b) do notcause a great aberration to the light of the wavelengths λ₂ and λ₃.Therefore, the optical path difference caused by each step of FIG. 11(b) is desirably 10M times (M is an integer other than 0) of thewavelength λ₁.

The steps shown in FIG. 11( c) preferably have a stair shape in whichthe height monotonically increases or decreases. In this case, a moldfor molding the objective lens element can easily be produced.

FIG. 12 is a diagram illustrating the objective lens element accordingto Embodiment 5. Steps 134 p and 134 q shown in FIG. 12 are the stepsshown in FIG. 11( b). FIG. 13 is a schematic plan view when FIG. 12 isviewed from below. A region having an effective range (or an effectivediameter) 134D is divided into three concentric regions, namely, anouter part 134F, an intermediate part 134B, and an inner part 134C. Itshould be noted that boundaries 134A, 134D, and 134E shown by brokenlines in FIG. 13 are imaginary boundaries, and it is unnecessary toactually provide boundary lines thereon.

On at least the inner part 133C, the stair shape shown in FIG. 11( c) isformed. When the light of the wavelength λ₁ is used, the objective lenselement 144 converges the first incident light 61 incident on the innerpart 134C, the intermediate part 134B, and the outer part 134F, andforms a spot on the information recording surface 91 of the optical disc9 through the base material having the thickness t₁. When the light ofthe wavelength λ₂ is used, the objective lens element 144 converges thesecond incident light 62 incident on the inner part 134C and theintermediate part 134B, and forms a spot on the information recordingsurface 101 of the optical disc 10 through the base material having thethickness t₂. When the light of the wavelength λ₃ is used, the objectivelens element 144 converges the third incident light 63 incident on theinner part 133C, and forms a spot on the information recording surface111 of the optical disc 11 through the base material having thethickness t₃. Here, NA₁ is equal to or higher than 0.85, NA₂ is equal toor higher than 0.6, and NA₃ is equal to or higher than 0.45.

In other words, the inner part 134C is a region shared by the light ofthe wavelengths λ₁, λ₂, and λ₃, the intermediate part 134B is a regionshared by the light of the wavelengths λ₁ and λ₂, and the outer part134F is a region dedicated for the light of the wavelength λ₁. A regionwhich is a combination of the inner part 134C and the intermediate part134B corresponds to the inner part 132B shown in FIG. 8. The outer part134F corresponds to the outer part 132F shown in FIG. 8. Any of theabove-described exemplary configurations of the outer part can beapplied to the outer part 134F.

In the objective lens elements according to Embodiments 1 and 3 to 5described above, the outer part is a region dedicated for the light ofthe wavelength λ₁, but may have an aspherical shape in which no stepsare formed. Alternatively, a sawtooth-like diffraction grating may beformed on the outer part, and the sawtooth height may be set to such aheight that an optical path difference of about 1 wavelength is providedto the light of the wavelength λ₁. Still alternatively, steps eachhaving such a height that an optical path difference which is about anintegral multiple of wavelength is provided to the light of thewavelength λ₁ may be formed on the outer part. Still alternatively, theheight of each step formed on the outer part may be such a height thatan optical path difference which is about N times (N is an integer) ofwavelength is provided to the light of the wavelength λ₁. In this case,N may be an integer other than multiples of 5. When N is an integerother than multiples of 5, the wavefront of the light of the wavelengthλ₂ is disturbed, and an aperture limiting effect can be obtained. When Nis an odd number, the wavefront of the light of the wavelength λ₃ isdisturbed, and an aperture limiting effect can be obtained. Due to sucha configuration, aperture limitation can be performed such that thelight of the wavelengths λ₂ and λ₃ incident on the outer partsubstantially does not contribute to spot formation on an informationrecording surface, and the numerical aperture can be set to a desiredvalue.

Embodiment 6

FIG. 14 is a diagram illustrating an optical head device according toEmbodiment 6. The optical head device includes a laser beam source 1which emits blue light of the wavelength λ₁ (390 nm to 415 nm: normally408 nm), a laser beam source 20 which selectively emits red light of thewavelength λ₂ (630 nm to 680 nm: normally 660 nm) and infrared light ofthe wavelength λ₃ (770 nm to 810 nm: normally 780 nm), a collimatinglens (first convex lens) 8, an upward reflection mirror 12 which bendsan optical axis, and the objective lens element 143. The optical disc 9is a third generation optical disc (e.g., BD) which has a base materialhaving a thickness t₁ (about 0.1 mm) and on which at least one ofrecording, reproducing, and erasing is performed with a light beam ofthe wavelength λ₁. The optical disc 10 is a second generation opticaldisc (e.g., DVD) which has a base material having a thickness t₂ (about0.6 mm) and on which at least one of recording, reproducing, and erasingis performed with a light beam of the wavelength λ₂. The optical disc 11is a first generation optical disc (e.g., CD) which has a base materialhaving a thickness t₃ (about 1.2 mm) and on which at least one ofrecording, reproducing, and erasing is performed with a light beam ofthe wavelength λ₃. In FIG. 14, only the base materials (portions fromlight incident surfaces to recording surfaces) of the optical discs 9and 10 are shown. In reality, each of the optical discs 9 and 10 isattached to a protective material in order to reinforce mechanicalstrength and to have a thickness of 1.2 mm which is the same as thethickness of CD. The optical disc 10 is attached to a protectivematerial having a thickness of 0.6 mm, and the optical disc 9 isattached to a protective material having a thickness of 1.1 mm. Theoptical disc 11 also has a thin protective material, but illustration ofthe protective materials is omitted for simplification.

In the present embodiment, the configuration employing thetwo-wavelength laser beam source 20 which emits the light of thewavelengths λ₂ and λ₃ is shown. However, a configuration in whichdifferent light sources are employed for these wavelengths, respectivelyand optical paths are combined by using a dichroic mirror is alsopossible.

The laser beam sources 1 and 20 are preferably semiconductor lasersources. When semiconductor laser sources are used, the optical headdevice and an optical disc recording/reproducing apparatus employingthis optical head device can be decreased in size, weight, and powerconsumption.

When any of recording, reproducing, and erasing is performed on thehighest-recording density optical disc 9, a light beam 61 of thewavelength λ₁ emitted from the laser beam source 1 is reflected by abeam splitter 4, is converted by the collimating lens 8 intosubstantially parallel light, and is further converted by a quarterwavelength plate 5 into polarized light. The quarter wavelength plate 5is designed to serve as a quarter wavelength plate for the light of bothwavelengths λ₁ and λ₂. The light emitted from the quarter wavelengthplate 5 is bent by the upward reflection mirror 12, and is converged bythe objective lens element 143 on the information recording surface 91through the base material of the optical disc 9. Here, for convenienceof illustration, the upward reflection mirror 12 is shown to bend thelight beam in the upward direction of the drawing. However, in reality,the upward reflection mirror 12 is disposed so as to bend the light beamoptical axis in a direction perpendicular to the sheet.

The light beam reflected by the information recording surface travelsalong the optical path in the reverse direction, is converted by thequarter wavelength plate 5 into linearly polarized light having apolarization plane different from an initial polarization plane, almosttotally passes through the beam splitter 4, and is totally reflected bya beam splitter 16. The light beam reflected by the beam splitter 16 isdiffracted by a detection diffraction element 31, then is converged by adetection lens 32, and is incident on a photodetector 33. Output of thephotodetector 33 is subjected to arithmetic processing to obtain a servosignal and an information signal which are used for focus control andtracking control. The beam splitter 4 includes a polarization splittingfilm which totally reflects linearly polarized light of the wavelengthλ₁ having a certain direction and totally passes linearly polarizedlight of the wavelength λ₁ having a direction orthogonal to the certaindirection. The polarization splitting film totally passes light beams ofthe wavelengths λ₂, and λ₃. As described above, the beam splitter 4 isan optical path branching element which has polarization properties aswell as wavelength selectivity. When the beam splitter 4 has nopolarization dependency, it is possible to omit the quarter wavelengthplate 5.

Next, when any of recording, reproducing, and erasing is performed onthe optical disc 10, a light beam 62 of the wavelength λ₂ which isemitted from the laser beam source 20 and is substantially linearlypolarized light passes through the beam splitter 16 and the beamsplitter 4 and is converted by the collimating lens 8 into substantiallyparallel light. The light beam emitted from the collimating lens 8 isbent by the upward reflection mirror 12 and is converged by theobjective lens element 143 on the information recording surface 101 ofthe optical disc 10.

The light beam reflected by the information recording surface travelsalong the optical path in the reverse direction, almost totally passesthrough the beam splitter 4, and is totally reflected by the beamsplitter 16. The light beam reflected by the beam splitter 16 isdiffracted by the detection diffraction element 31, then is converged bythe detection lens 32, and is incident on the photodetector 33. Outputof the photodetector 33 is subjected to arithmetic processing to obtaina servo signal and an information signal which are used for focuscontrol and tracking control. In order to obtain a servo signal for theoptical disc 9 and a servo signal for the optical disc 10 by using thephotodetector 33 shared by the wavelengths λ₁ and λ₂, the light-emittingpoint of the laser beam source 1 and the light-emitting point (the lightof the wavelength λ₂) of the laser beam source 20 are located in aconstruct-image relation with respect to a common position on theobjective lens element 143 side. When such a configuration is provided,the number of detectors and the number of wires can be reduced.

The beam splitter 16 includes a polarization splitting film whichtotally passes linearly polarized light of the wavelength λ₂ having acertain direction and totally reflects linearly polarized light having adirection orthogonal to the certain direction. The polarizationsplitting film totally passes a light beam of the wavelength λ₁. Asdescribed above, the beam splitter 16 is also an optical path branchingelement which has polarization properties as well as wavelengthselectivity. When the beam splitter 16 has no polarization dependency,it is possible to omit the quarter wavelength plate 5.

The operation of each component and the optical path when any ofrecording, reproducing, and erasing is performed on the optical disc 11are the same as those in the case of the optical disc 10, except thatthe wavelength of the light emitted from the laser beam source 20 isdifferent. Thus, the repeated description is omitted.

In the example of FIG. 14, in the objective optical system, theobjective lens element 143 described in Embodiment 4 is used, but theobjective lens element 144 according to Embodiment 5 may be used.

In the objective optical system, the objective lens element 141according to the Embodiment 1 or the objective lens element 142according to Embodiment 3 may be used. In this case, a light sourcewhich emits only light of the wavelength λ₂ can be used instead of thelaser beam source 20, and an optical head device which enablescompatibility with the optical disc 9 and the optical disc 10 can beconfigured.

Further, in the objective optical system, the objective lens elementaccording to Embodiment 2 may be used. In this case, an optical headdevice dedicated for the optical disc 9 can be configured withoutproviding the laser beam source 20. Alternatively, an optical headdevice may be configured by combining the objective lens elementaccording to Embodiment 2 and an objective lens element which enablescompatibility with light of the wavelengths λ₂ and λ₃.

Additional components of the optical head device will be describedbelow.

A three-beam grating (diffraction element) 3 may be disposed between thelaser beam source 1 and the beam splitter 4. Due to this configuration,it is possible to detect a tracking error signal of the optical disc 9by the well-known differential push-pull (DPP) method.

A relay lens 2 may be disposed between the laser beam source 1 and thebeam splitter 4. Due to this configuration, the numerical aperture onthe collimating lens 8 side of the light beam 61 of the wavelength λ₁can be set to an appropriate value.

A three-beam grating (diffraction element) 22 may be disposed betweenthe laser beam source 20 and the beam splitter 16. Due to thisconfiguration, it is possible to detect a tracking error signal of theoptical disc 10 by the well-known differential push-pull (DPP) method.

It is effective to change the parallelism of the light beam by movingthe collimating lens 8 along the optical axis direction (the right-leftdirection in FIG. 14). There are errors in the thicknesses of the basematerials. In addition, when the optical disc 9 is a multilayer disc, abase material having a predetermined thickness is provided betweenrecording layers. A spherical aberration occurs due to an error of thebase material thickness and the base material between the recordinglayers, but can be compensated by moving the collimating lens 8 alongthe optical axis direction.

Spherical aberration compensation performed by moving the collimatinglens 8 is possible with about several hundreds mλ when the NA is 0.85,and a spherical aberration corresponding to increase/decrease of ±30 μmin base material thickness can also be compensated. In addition, whenrecording, reproducing, or erasing is performed on the optical disc 11by using the light beam of the wavelength λ₃, the collimating lens 8 maybe moved toward the left side of FIG. 14, namely, toward the laser beamsource 20. In this case, the collimating lens 8 converts a light beamtravelling toward the objective lens element 143, into diverging light,can move a convergence spot on the optical disc 11 further away from theobjective lens element 143, and can compensate a part of an aberrationcaused by the base material thickness. When the part of the aberrationis compensated by the collimating lens 8, an aberration compensationamount required for the objective lens element 143 can be reduced. Inaddition, the number of formed steps can be reduced and hence it can bemade easy to produce the objective lens element 143.

The beam splitter 4 may pass a portion (e.g., about 10%) of the linearlypolarized light of the wavelength λ₁ emitted from the laser beam source1. The passed portion of the light beam is guided by a converging lens 6to a photodetector 7. A signal obtained from the photodetector 7 is usedto monitor change in amount of the light emitted by the laser beamsource 1, and the change in amount of the light is fed back, wherebycontrol can be performed to keep the amount of the light emitted by thelaser beam source 1, constant.

The beam splitter 4 may reflect a portion (e.g., about 10%) of thelinearly polarized light emitted from the laser beam source 20. Thereflected light beam is guided by the converging lens 6 to thephotodetector 7. A signal obtained from the photodetector 7 is used tomonitor change in amount of the light emitted by the laser beam source20, and the change in amount of the light is fed back, whereby controlcan be performed to keep the amount of the light emitted by the laserbeam source 20, constant.

Embodiment 7

FIG. 15 is a diagram illustrating an optical information apparatusaccording to Embodiment 7.

The optical disc 9 is retained on a turntable 182 by a clamper 163 andis rotated by a motor 164 (the same applies to the optical discs 10 and11). An optical head device 155 is the optical head device described inEmbodiment 6 and includes any of the objective lens elements describedin Embodiments 1 to 5. The optical head device 155 is driven by a driveunit 151 to a position which faces a desired track on the optical disc 9(10, 11).

The optical head device 155 detects a positional relationship with theoptical disc 9 and outputs a focus error signal or a tracking errorsignal to an electric circuit 153. In response to the signal, theelectric circuit 153 sends a signal for finely adjusting the position ofthe objective lens element. On the basis of the output signal from theelectric circuit 153, the optical head device 155 performs focus controlor tracking control and performs recording, reproducing, or erasing ofinformation.

The optical information apparatus 167 according to the presentembodiment employs the optical head device 155 described in Embodiment6, and thus is compatible with a plurality of optical discs havingdifferent recording densities. Therefore, the optical informationapparatus 167 can be configured to be small in size and weight at lowcost.

Embodiment 8

FIG. 16 is a diagram illustrating a computer according to Embodiment 8.

The computer 300 includes the optical information apparatus 167according to Embodiment 7, an arithmetical unit 364 such as a centralprocessing unit (CPU). The computer 300 is connected to an input device365 for performing input of information, such as a keyboard, a mouse,and a touch panel, and an output device 181 such as a printer and adisplay device displaying information. The computer 300 performs anoperation on the basis of information inputted through the input device365, information read out from an optical disc through the opticalinformation apparatus 167, or the like, and outputs the result of theoperation to the output device 181.

Embodiment 9

FIG. 17 is a diagram illustrating an optical disc player according toEmbodiment 9.

The optical disc player 321 includes the optical information apparatus167 according to Embodiment 7, a decoder 366 which converts aninformation signal obtained from the optical information apparatus 167into image information, and a liquid crystal monitor 320. The opticaldisc player 321 can also be used as a car navigation system.Alternatively, an optical disc player which does not include a liquidcrystal monitor may be configured.

Embodiment 10

FIG. 18 is a diagram illustrating an optical disc recorder according toEmbodiment 10.

The optical disc recorder 110 includes the optical information apparatus167 according to Embodiment 7 and an encoder 368 which converts imageinformation into information for recording. The optical disc recorder110 preferably further includes the decoder 366 which converts aninformation signal obtained from the optical information apparatus 167into image information. By providing the decoder 366 and the encoder368, the optical disc recorder 110 can be configured to be able toperform both recording and reproducing. In FIG. 18, the optical discrecorder 110 is connected to an output device 361 such as a displaydevice and a printer, but may be integrated with the output device 361.

Embodiment 11

FIG. 19 is a diagram illustrating an information server according toEmbodiment 11.

The information server 336 includes the optical information apparatus167 according to Embodiment 7, a changer 334 which accommodates aplurality of optical discs and inserts an optical disc into the opticalinformation apparatus 167 or takes out an optical disc from the opticalinformation apparatus 167, and an input/output terminal 369. Theinput/output terminal 369 is a wired or wireless interface used forintroducing information to be recorded on an optical disc and outputtinginformation read from an optical disc to the outside. The informationserver 336 is connected to a network or another apparatus (e.g., acomputer, a phone, or a television tuner) via the input/output terminal369, and can exchange information therewith. The information server 336may be connected to the output device 361 such as a display device and aprinter. Alternatively, the information server 336 may not necessarilyinclude the changer.

Embodiment 12

FIG. 20 is a diagram illustrating a vehicle according to Embodiment 12.

The vehicle shown in FIG. 20 includes a body 233, the opticalinformation apparatus 167 according to Embodiment 7, an accommodationsection 239 which accommodates a plurality of optical discs, and achanger 238 which inserts an optical disc into the optical informationapparatus 167 or takes out an optical disc from the optical informationapparatus 167. On the body 233, a power generation section 234 whichgenerates power, a fuel storage section which stores a fuel supplied tothe power generation section 234, and a power source 236 are mounted. Inthe case of a train or an automobile, the vehicle further includeswheels 235 for running. In addition, when the vehicle is an automobile,the vehicle includes a steering wheel 230.

Since the vehicle according to the present embodiment includes thechanger 238 and the optical disc accommodation section 239, many opticaldiscs can readily be used. When an arithmetical unit 364 which processesinformation obtained from an optical disc to convert the informationinto an image, a semiconductor memory 237 which temporarily storesinformation, and a display device 242 are mounted on the body 233, it ispossible to reproduce video information recorded on an optical disc. Inaddition, when an amplifier 240 and a speaker 241 are provided in thebody 233, it is possible to reproduce sound or music recorded on anoptical disc. When a position sensor such as a GPS 232 is provided, acurrent position or a travelling direction can be displayed as an imageon the display device 242 in combination with map information reproducedfrom an optical disc, or can be outputted as sound from the speaker 241.When a wireless communication section 250 is provided, information canbe obtained from the outside and used with information of an opticaldisc in a complementary manner.

The present invention can be used in a BD/DVD-compatible orBD/DVD/CD-compatible optical head device.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It willbe understood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

What is claimed is:
 1. An objective lens element which has opticallyfunctional surfaces on an incident side and an exit side, whichconverges a first incident light beam of a wavelength λ₁ through a baseplate having a thickness t₁ to form a spot, and which converges a secondincident light beam of a wavelength λ₂ longer than the wavelength λ₁through a base plate having a thickness t₂ larger than the thickness t₁to form a spot, wherein at least either one of the optical functionsurfaces is a refractive surface which deflects the first and secondincident light beams by refractive power all over the surface, the atleast either one of the optical function surfaces being divided into aninner part which includes a rotational symmetry axis and through whichthe first and second incident light beams that substantially contributeto spot formation pass, and an outer part which is a ring-shaped regionsurrounding the inner part and through which only the first incidentlight beam that substantially contributes to spot formation passes, andthe at least either one of the optical function surfaces having aplurality of discontinuous steps on the inner part, and the plurality ofsteps change in height in the same direction from the optical axistoward the outer part, and each of the steps causes a constant opticalpath difference longer than the wavelength λ₁ to the first incidentlight beam and causes a constant optical path difference shorter thanthe wavelength λ₂ to the second incident light beam.
 2. The objectivelens element according to claim 1, wherein each of the steps provides anoptical path difference of about 1.25 wavelengths to light of thewavelength λ₁ and provides an optical path difference of about 0.75wavelength to light of the wavelength λ₂.
 3. The objective lens elementaccording to claim 1, wherein the outer part has an aspherical shape inwhich no steps are formed.
 4. The objective lens element according toclaim 1, wherein a diffraction grating having a sawtooth shape is formedon the outer part, and the sawtooth shape has such a height that anoptical path difference of about 1 wavelength is provided to light ofthe wavelength λ₁.
 5. The objective lens element according to claim 1,wherein steps each having such a height that an optical path differencewhich is substantially an integral multiple of wavelength λ₁ is providedto light of the wavelength λ₁ are formed on the outer part.
 6. Theobjective lens element according to claim 5, wherein each step on theoutermost portion of the outer part has such a height that an opticalpath difference which is substantially N times (N is an integer otherthan multiples of 5) of wavelength λ₁ is provided to the light of thewavelength λ₁.